Meanwhile, fighting continued in eastern Ukraine, with reports of an attack on a border post in Donetsk region.

A convoy of vehicles carrying pro-Russian separatists was involved in the attack near the Marynivka checkpoint which has now been repelled by government forces, the Ukrainian border guard service said.

The attack comes a day after rebels seized a border guard base in neighbouring Luhansk region after days of combat.

'Resolute message'

Speaking at a news conference in Brussels with Mr Cameron, Mr Obama said Russia should "seize the opportunity" provided by the change of leadership in Kiev, and be prepared to face sanctions if the situation continued to deteriorate.

"Russia needs to recognise that President-elect Poroshenko is the legitimately elected leader of Ukraine and engage the government in Kiev," he said.

"Given its influence over the militants in Ukraine, Russia continues to have a responsibility to convince them to end their violence, lay down their weapons and enter into a dialogue with the Ukrainian government.

"On the other hand, if Russia's provocations continue, it's clear from our discussion here that the G7 nations are ready to impose additional costs on Russia."